This invention relates to DC dynamoelectric machines and more particularly to one in which high currents are transmitted through brushes.
Conventional DC machines utilize electrographic or carbon brushes that carry a maximum current density of 50 to 100 amps per square inch with a corresponding voltage drop of approximately 1 volt. Typically conventional DC machines include small interpoles and interpole windings, located between the main stator poles used to generate voltages in the conductors undergoing commutation such that the conductor current reverses in a nearly linear manner. This is called linear commutation. Under these conditions, current and power densities are distributed uniformly throughout the brush volume. Slight deviations from ideal linear commutation produces non-uniform currents and power densities, which are easily accommodated by the bulk brush material.
A new generation of low voltage high current DC machines currently being developed requires current transfers through sliding contacts at 1,000 to 2,000 amps per square inch with a voltage drop of less than 200 millivolts. Such current transfer characteristics may be achieved by utilizing metal-graphite brushes operating either in air or possibly in a controlled atmosphere of humidified carbon dioxide. During a current switching interval, however, when the rotor bar current diminishes from the normal operating level to zero, the rotor bar leakage inductance induces an additional bar to brush voltage that tends to maintain a constant bar current. As the brush to bar contact area diminishes and the corresponding contact resistance increases, the contact power density increases dramatically from several hundred to several thousand or more watts per square inch. At such power densities along the brush to bar interface, the metal constituent of the metal graphite brushes melts and deposits on the collector bar surface. Thus, a thin depletion zone forms on the brush surface beginning at the trailing end and extending toward the leading end. The extent of the depletion appears to be a complicated function of surface filming, power density, and brush motion.
Such depletion is undesirable because of several reasons:
1. Brush contact resistance and hence brush voltage drop is increased. PA1 2. Substantial metal depletion on the conductor bar leads to poor mechanical contact thereby increasing interface resistance. Deposition build-ups in the insulating gap between adjacent collector bars may lead to shorting of the bars. PA1 3. Switching characteristics determined by depletion are highly load dependent due to the sensitivity to energy stored in rotor bar leakage inductance. Hence, trailing edge arcing at a given load current may or may not occur depending on the extent of depletion existing at the time of establishing the load current. PA1 4. Due to the variable brush characteristics associated with depletion formation, machine performance predictions are at best approximations.
Although trailing edge arcing can be effectively eliminated by using a conventional electrographic brush at the trailing edge to absorb excessive bar leakage energy, this technique has not provided an entirely satisfactory solution to the depletion problem. Depletion of the metal graphite brushes still occur when used with a trailing edge electrographic brush. In addition, high power dissipation within the electrographic brush increases that brush's body temperature to unacceptable levels.